1. Field of the Invention
This invention relates to a memory device having a plurality of circulating storage loops, such as a magnetic bubble memory.
2. Description of the Prior Art
In a magnetic bubble memory device which exploits bubble domains formed in magnetic thin films, a large number of storage loops are arrayed in parallel on each bubble chip, and information is written into or read out from bits of the respective storage loops having the same addresses in parallel.
In such a magnetic bubble memory device, the proportion of the cost of the bubble chips to that of the whole device is large. It is, therefore, important to enhance the yield of the bubble chips.
In a case where bubble chips having bad or defective storage loops are not assembled into the device when such defective chips have been produced in large numbers, the available percentage lowers noticeably. It is, therefore, common practice to enhance the available percentage by assembling the chips which include the defective storage loops.
In this case, however, a control arrangement must be provided to prevent the use of the defective storage loops for write and read on the basis of information indicating which storage loops of the bubble chip are defective.
In one prior art system having such a control arrangement, the positional information of the defective storage loops within the bubble chip is stored in an external read only memory (ROM) in advance. With this system, however, the read only memories need to be disposed in correspondence with the respective bubble chips. This renders the device very expensive. Moreover, the relationship between the bubble chips and the corresponding ROMs must be reliably managed, and the device cannot be used at all when the corresponding relations have been disordered.
There have, accordingly, been proposed other prior art systems wherein at least one special storage loop is disposed among a large number of storage loops and the positional information of the bad or defective storage loops is written in the special loop or loops in series or in parallel. This eliminates the problems of the management of the positional information of the defective storage loops.
In one of these systems, a plurality of special storage loops are disposed for the defective storage loop-positional information, and the succession of defective storage loop-positional information is written in parallel at the same addresses of these special storage loops. The same positional information as the aforecited information is written over all the addresses, and the positional information of the defective storage loops can be read during every operation carried out by assigning any desired address. This system can dispense with the external memories. On the other hand, assuming by way of example that the number of permissible defective loops is 4 and that the number of bits of the positional information is 8, 32 special storage loops must be disposed for storing the defective storage loop-positional information. Therefore, the bubble chip itself becomes larger in size and higher in cost. Moreover, it is necessary to secure a bubble chip in which all of the special loops within a specified area thereof are good or non-defective loops. Therefore, the system is not very useful for the enhancement of the yield.
In another prior art system, one or more storage loops for storing the defective storage loop-positional information are specially disposed within the bubble chip, and the positional information of the defective storage loops are successively written into the loop or loops in series. With this system, the number of the special loops decreases. On the other hand, in order to read out the defective storage loop-positional information and store them in a writable and readable random access memory (RAM) in advance, it is necessary to successively read out all of the information of the bubble chip to which the special loop belongs and to select the information from the special loop from among the read information. This leads to the disadvantage that a long time is required. It is, therefore, desirable to read out only the required information of the special loop and write it into the memory. In such a case, however, a special circuit is required, which results in the disadvantage of a complicated processing circuit.